To obtain a basis for the evaluation of postimplantation changes in bioprostheses made of parietal pericardium, comparative histologic, scanning and transmission electron microscopic studies were made of the structure of: 1) normal bovine parietal pericardium; 2) glutaraldehyde- treated pericardial patches to be used for repair of cardiac defects, and pericardial tissue cusps of unimplanted Ionescu-Shiley valves.